EP0516164B1

EP0516164B1 - Watch exterior part

Info

Publication number: EP0516164B1
Application number: EP92109123A
Authority: EP
Inventors: Nobuyuki C/O Itami Works Of Sumitomo Kitagawa; Toshio C/O Itami Works Of Sumitomo Nomura; Yoichi c/o Naimiki Precision Jewel Co. L Yaguchi; Hidehiro c/o Naimiki Precision Jewel Co. Uchiumi; Naoko c/o Naimiki Precision Jewel Co. Iwashimizu
Original assignee: Namiki Precision Jewel Co Ltd; Sumitomo Electric Industries Ltd
Current assignee: Namiki Precision Jewel Co Ltd; Sumitomo Electric Industries Ltd
Priority date: 1991-05-31
Filing date: 1992-05-29
Publication date: 1999-10-27
Anticipated expiration: 2012-05-29
Also published as: DE69230194D1; JPH04354839A; KR970011257B1; DE69230194T2; KR920022063A; EP0516164A1; US5403374A

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to watch exterior parts having complicated shapes and formed of an alloy corresponding to stellite.

Description of the Background Art

Recently, hard materials which do not easily get hurt and have excellent durability are used for watch exterior parts such as watch frames, watch band pieces and the like, and especially there is a tendency that cemented carbide including WC, TaC, TiC and so forth, or alloys corresponding to stellite containing Co-Cr-W, for example, are widely used.
Such cemented carbide or alloys corresponding to stellite have texture in which hard particles such as carbide, nitride or carbonitride of W, Ta, Ti, Cr, for example, are bound with metal of the iron group such as Co, Fe, Ni and the like, which are manufactured by conventionally well-known powder metallurgy. That is, they are manufactured by a method of mixing WC powder, TaC powder, Co powder, Ni powder etc. according to a predetermined alloy composition, molding the material powder of alloy by pressing, and sintering obtained molded bodies.
However, since molded bodies are obtained by pressing in the above-described normal powder metallurgy, there have been problems such as limitation of shapes of manufactured products and dimensional precision, for example. That is, products having shapes which can be formed in one axis direction only can be manufactured in the die compaction. Even if the CIP (Cold Isostatic Press) capable of forming three-dimensional shapes is used, excellent precision could not be expected because they are molded inside rubber molds. Accordingly, watch exterior parts have been conventionally obtained by manufacturing sintered bodies having simple shapes by the normal powder metallurgy, applying secondary operation to the sintered bodies to realize complicated shapes such as various kinds of watch frames and watch band pieces having three dimensional curved surfaces and small holes, and applying surface finishing by polishing in order to improve decorativeness to portions such as surfaces of exterior parts as needed.
However, because cemented carbide and alloy corresponding to stellite are extremely difficult to be worked, they can be worked only by grinding with diamond grinding stones or discharging in the secondary operation. Especially, discharge operation has been essential in formation of three-dimensional curved surfaces of inner surfaces of watch frames and interior surfaces, small holes for provision of stems, and the like. However, if sintered bodies of cemented carbide or alloys corresponding to stellite are subjected to discharge process, processed surfaces are transferred to be brittle over a depth of approximately 5 through 100µm due to removal, oxidation or the like of metallic constituents to decrease the material strength, which is likely to cause small breakage from the processed surface due to impact applied from outside leading to breakage of the entirety of a watch exterior part.
Accordingly, watch exterior parts formed of cemented carbide or alloy corresponding to stellite have been conventionally made into simple shapes to reduce discharge operation as possible, or the thickness thereof has been designed to be larger than needed in order to maintain the strength after the discharge operation. Therefore, it had disadvantages such that designs of watch frames and watch bands are limited, and total weight of watch increases, for example.
From JP-A-5 130 538 it is known that watch cases can be made from stellite. From EJ R.M. German,
Powder Injection Molding", Metal Powder Industries Federation, Princeton, New Jersey, 1990, it is known that powder injection molding may be used for forming small parts of complex shape.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide watch exterior parts formed of cemented carbide or alloy corresponding to stellite having complicated shape such as three-dimensional curved surfaces and small holes and also having high strength without applying secondary operation such as discharge operation.
In order to achieve the above-mentioned object a watch exterior part according to claim 1 or claim 3 is provided. In a method of manufacturing watch exterior parts of the present invention, organic binder is mulled into material powder of an alloy corresponding to stellite, it is then injection-molded into shape of watch exterior parts having a three-dimensional curved surface or a small hole, and sintered after removing organic binder from an obtained molded body.
The alloy corresponding to stellite consists of the components Co, W and CrC.
A watch exterior part of the present invention manufactured according to the method is characterized in that it is formed of a sintered body and has a three-dimensional curved surface of as-sintered surface or has a small hole of as-sintered surface on its inner surface, and may have a three-dimensional curved surface of a polished surface obtained by polishing the as-sintered surface in order to enhance decorativeness.
External parts for watch having three-dimensional curved surfaces include watch frame pieces and watch band pieces, for example. As small holes of watch exterior parts include band attachment holes, stem attachment holes, band connecting holes, for example.
In the method for forming such watch exterior parts the injection molding which has been conventionally used in manufacturing plastic products and also recently used in manufacturing ceramics products is applied to the powder metallurgy of an alloy corresponding to stellite to produce watch exterior parts such as watch frames and watch band pieces having complicated shape. That is, a molded body having complicated shape with a three-dimensional curved surface, a small hole or the like which is of a similar figure to that of a watch exterior part is formed using injection molding from material powder into which organic binder is mulled, and the molded body is subjected to a binder removing process and then sintered to obtain a watch exterior part having a predetermined complicated shape.
The material powder is mixed and pulverized simultaneously in a dry or wet manner using a general ball mill or Attoritor, a high energy ball mill developed by Attoritor Union Process Inc. If the mixing and pulverizing are not enough, the sintering characteristics are degraded and a sintered body which is close to true density can not be obtained. Accordingly, the material powder after mixing and pulverizing contains particles with size equal to or smaller than 20µm by 20 weight % or more.
As organic binder to be mulled into material powder, one which has been conventionally used in injection molding of ceramics products can be used, where, for example, polyethylene, polypropylene, polystyrene, acrylic, ethylene-viniyl acetate, various kinds of wax, paraffin and so forth can be used singly or in combination.
In the binder removing process, organic binder is melted and flowed out, or decomposed or sublimated by heating molded bodies according to types of mulled organic binder. Since specific gravity of molded bodies may be different, attention must be paid to suppress deformation due to self-weight. Also, the atmosphere of binder removing process is preferably vacuum, or non-oxidizing gas such as hydrogen gas, nitrogen gas or inactive gas in order to suppress oxidation of material powder.
By sintering a molded body subjected to binder removing process in vacuum or hydrogen gas, a sintered body with complicated shape configuring a predetermined exterior for watch can be obtained. The sintering temperature can be the same as that in powder metallurgy using normal pressurizing molding. However, attention must be paid because deformation is apt to be caused in molded bodies if the sintering temperature is too high, and sintering is preferably performed in a temperature range of approximately +50°C from a melting point of metal phase of Co, for example.
As described above, in the method of the present invention, molded bodies obtained by injection molding are sintered, and sintered bodies of alloy corresponding to stellite can be obtained having complicated shape with three-dimensional curved surfaces, small holes and the like. Without necessity of secondary operation such as discharge operation, watch exterior parts such as watch frames and watch band pieces and the like having three-dimensional curved surfaces of as-sintered surfaces or small holes of as-sintered surfaces on inner peripheral surfaces can be obtained as they are. It is a matter of course that surface finishing can be normally applied in order to implement a mirror surface state or the like by slightly polishing at portions where decorativeness must be enhanced such as surfaces of exterior parts. Accordingly, substantial secondary operation such as discharge operation is not required in order to form configurations of watch exterior parts.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a plan view illustrating one specific example of a component piece of watch band according to the present invention.
Fig. 2 is a plan view illustrating watch frame pieces which include two in one set as one specific example of a watch frame according to the present invention.
An embodiment of the present invention will be described below.
Examples for forming a watch exterior part made of an alloy different from the alloy according to the present invention are presented first, however.

Example 1:

88 weight % of WC powder having mean particle diameter of 1µm and 12 weight % of Ni powder with mean particle diameter of 2µm were pulverized and mixed for 30 hours in ethyl alcohol using a ball mill. After drying obtained mixed powder, 5 weight % of paraffin and 2 weight % of polyethylene were added as organic binder and mulled for two hours by a kneader. The mulled substance was injection-molded into a mold for watch band pieces by an injection molding machine. Next, the molded body is heated to 450°C at a temperature increasing rate of 20°C/hour in nitrogen gas and held for one hour to remove the organic binder.
Sintering the molded body after the binder removing process in a vacuum state at 1400°C for 30 minutes, component pieces 1 for watch band having three dimensional curved surfaces and two kinds of assembly holes 2 with diameters of 0.8mm and 1.0mm as shown in Fig. 1 were manufactured.
As a result of measuring diameters of assembly small holes 2 of the watch band pieces 1, it was known that the precision is ±0.05mm in hole diameter and ±0.08mm in hole pitch, which means that precision enough in assembly can be obtained without performing conventional secondary operation such as discharge operation, and products can be manufactured only by mirror surface finishing by burr removing on surfaces and polishing process of surfaces.
Furthermore, at least 0.75mm of thickness of a wall of a small hole has been conventionally necessary for obtaining predetermined strength in watch band pieces in which assembly holes are formed by discharge operation. However, it was revealed that the thickness can be made thinner to 0.5mm according to the present invention, so that the degree of freedom in designing increased and it became applicable to design with small thickness and small weight.
Furthermore, measurement of roughness (Rmax) of an as-sintered surface of component piece of watch band 1 revealed that Rmax of a product manufactured by conventional powder metallurgy using mold press was 5µm, but Rmax according to the above example was 2µm, which is extremely smooth, and it was known that the number of processes required in polishing step for surface finishing can be largely reduced.

Example 2:

By the method similar to example 1, a watch frame piece (nine o'clock side) 3, of two watch frame pieces, formed of cemented carbide with a composition of 88 weight % of WC - 12 weight % of Ni and having a three-dimensional curved surface which is to be fixed to a ring 6 made of stainless material as shown in Fig. 2 was manufactured. Also, for comparison, a press molded body of material powder having composition the same as one described above was sintered under the same condition, and then a watch frame piece 3 having the same configuration was manufactured with a three dimensional curved surface formed by discharge operation.
Strength test has been made in which load is applied in the arrow A - A direction of Fig. 2 about each of obtained four watch frame pieces of example 2 and the comparative example, and the measured results of breaking load are shown in table 1.

watch frame piece breaking load (kg)

example 2 88, 69, 89, 88

comparative example 45, 38, 47, 44
From table 1, it is known that watch frame pieces of example 2 have strength approximately double that in the comparative example. Also, it was observed that breakage in the comparative example all took place starting at planes subjected to discharge operation.

Embodiment of the present invention:

By the method similar to example 1, a watch frame piece (three o'clock side) 4, of two watch frame pieces to which a ring 6 made of stainless material is to be attached as shown in Fig. 2, having a three-dimensional curved surface and a small hole 5 for provision of stem with diameter of 1.5mm was manufactured. Changing material powders, a watch frame piece 4a was formed of cemented carbide with composition of 70 weight % of Tic - 10 weight % of Mo₂C - 20 weight % of Ni for comparison, a watch frame piece 4b was formed of cemented carbide of 90 weight % Tac - 10 weight % of Ni also for comparison, and a watch frame piece 4c was formed according to the present invention of alloy corresponding to stellite with composition of 50 weight % of Co - 40 weight % of CrC - 10 weight % of W.
For further comparison, material powder with composition the same as each of the above-mentioned watch frame pieces 4a, 4b, 4c was injection-molded into molded bodies having no small holes for provision of stems, and the molded bodies were sintered under conditions the same as above to form watch frame pieces 4 (having no small hole 5 for provision of stem) of Fig. 2. Subsequently, small holes 5 for provision of stem only was formed by discharge operation to manufacture each of watch frame pieces 4d (including WC-Ni), 4e (including TaC-Ni) and 4f (alloy corresponding to stellite) of a comparative example having the same configuration as that of watch frame piece 4 of Fig. 2.
A strength test has been applied to each of the obtained watch frame pieces of the present invention and the comparative examples in which load is applied in the arrow direction of Fig. 2, and the results of measuring breaking load are shown in table 2.
It was observed that breakage in watch frame pieces 4d-4f in the comparative example all took place starting at inner peripheral surfaces of small holes formed by discharge operation.
According to the embodiment described above, watch exterior parts such as watch frames and watch band pieces with high strength can be provided which have three dimensional curved surfaces formed of as-sintered surfaces or formed by surface polishing of as-sintered surfaces, small holes of as-sintered surfaces and the like, and which are formed of an alloy corresponding to stellite and consisting of CrC, W and Co.
Furthermore, there is no need of secondary operation such as discharge operation, and the as-sintered surfaces are smooth, so that working steps can be largely simplified as compared to conventional cases. Furthermore, since high strength can be accomplished even with small thickness, it is possible in watch exterior parts of the present invention to largely improve and modify design.

Claims

A watch exterior part which is formed of sintered alloy consisting of the components CrC, W and Co, has a three-dimensional curved surface of an as-sintered surface and wherein said alloy is integrally molded to a shape of the watch part by injection molding of mixture of said metal powder into which an organic binder is mulled and the molded alloy is subjected to a binder removing process in vacuum or non-oxidizing gas and wherein the metal powder contains particles with particle size of 20 µm or smaller by 20 wt-% or more before sintering.
The watch exterior part according to claim 1, further comprising a three-dimensional curved surface of a polished surface obtained by polishing an as-sintered surface.
A watch exterior part which is formed of sintered alloy consisting of the components CrC, W and Co and has a small hole with an as-sintered surface at the inner peripheral surface wherein said alloy is integrally molded to a shape of the watch part by injection molding of mixture of said metal powder into which an organic binder is mulled and the molded alloy is subjected to a binder removing process in vacuum or non-oxidizing gas and wherein the metal powder contains particles with particle size of 20 µm or smaller by 20 wt-% or more before sintering.